1. Field of the Invention
This invention relates generally to driving light emitting diodes (LEDs), and more specifically to efficient duty cycle control of multiple LEDs in a series connected string of such LEDs, so as to control brightness of individual LEDs and color mixture of groups of LEDs.
2. Description of the Related Art
LEDs are widely used in lighting and display applications due to their high efficiency, long life, ruggedness and compact size. Early LEDs emitted red light, and found usage in calculator and watch displays. The availability of green, yellow and orange LEDs widened the usage of LEDs, but it was the development of blue and white light LEDs which led to widespread usage in backlight, flashlight, architectural and automotive applications. As LED power and efficiency increases, they are finding increased usage in architectural and general indoor and outdoor lighting applications.
One type of white LED uses a blue or ultraviolet emitting die with a phosphor deposited on or in proximity to the die. The mix of direct light from the die and light emitted by the excited phosphor blends to generate the color of such LEDs. Such white LEDs have the advantage of a single die with simple current drive requirements, but the color temperature and subjective quality of the white light so generated may not be optimum for the intended use.
Another way of generating a wide range of colors using LEDs is to mix the primary colors red, green and blue emitted from individual red emitting, green emitting, and blue emitting die or packaged LEDs. By controlling the relative current in each LED, and hence the relative brightness of each, a wide range of colors of varying brightness may be generated. Because the LED die or packaged LEDs are compact, a triad of red, green, and blue LEDs may be made small enough to be used as a single pixel in an array of pixels in a large display. Such LED array displays have found wide usage in applications requiring high brightness, such as stadium displays and signs along roadways.
In such an array application, a large number of LEDs are controlled by drivers to modulate the intensity of each LED. For example, to provide medium quality resolution, an array having 320 by 240 pixels may be used, each pixel having a red, green and blue LED, for a total of about 230,000 LEDs and drivers. In some applications, time division multiplexing techniques may be used to reduce the number of drivers. For example, a single row of pixels at a time may be turned on, sequentially addressing each row much like television scan lines. However, such time division multiplexing decreases overall brightness, so is not usable in all applications.
A large array of diodes and their drivers dissipates a considerable amount of power. For example, if each LED in a non-multiplexed 320 by 240 pixel display is driven with 40 ma of current at a typical voltage drop of 2.5 volts, each LED dissipates 0.1 watts, for a total array dissipation of nearly 23 kw with all LEDs on. Typical display dissipation with average image content is significantly less, but if linear drivers are used to control the current and brightness of each LED, many kilowatts of power, split between the LEDs and the drivers, may still be dissipated.
Yet another application of red, green, and blue LEDs is in sequential lighting for video projection. Some video projection systems, such as those using an array of micro mirrors, generate a color image by sequentially illuminating the mirror array with red, green and blue light. Current practice is to use a color wheel having red, green and blue filters between a white projection lamp and the mirror array, rapidly spinning the color wheel in synchronization with display of red, green and blue portions of the image on the mirror array. For some applications such as micro projection displays to be used in cell phones and other small electronic devices, LEDs are a more efficient light source. By utilizing sequentially switched red, green, blue LEDs, the color wheel and its associated mechanical complexity may be eliminated.
A more efficient switch-mode driver is therefore desirable, which has lower dissipation in the driver at all LED brightness levels, and which is better suited to sequential lighting of LEDs.